The present invention relates to an air-fuel ratio feedback control method and apparatus of an internal combustion engine, more specifically to an air-fuel ratio feedback control method and apparatus using an electrical digital computer.
An internal combustion engine, in general, emits gases containing pollutants such as carbon monoxide (CO), nitrogen oxides (NOx), and unburned or partly burned hydrocarbons (HC). When these pollutants are to be cleaned using a three-way catalytic converter, it is required to highly and precisely control the air-fuel ratio within a range around the stoichiometric air-fuel ratio such that all of the three components, i.e., CO, NOx, and HC can be removed effectively.
Therefore, the internal combustion engine employing the above-mentioned three-way catalytic converter usually adopts a method of controlling the feedback of air-fuel ratio responsive to signals from a concentration sensor (exhaust gas sensor) which detects the concentrations of particular components in the exhaust gas. Among many concentration sensors, an oxygen concentration sensor (hereinafter referred to an O.sub.2 sensor) for detecting the oxygen concentration has been extensively used for automobiles, such as a stabilized zirconia element or a titania element. When the air-fuel ratio in the atmosphere hovers around 14.5 (stoichiometric air-fuel ratio) this type of O.sub.2 sensor suddenly changes in electric properties. More specifically, the O.sub.2 sensor detects the changes in the air-fuel ratio which causes the electric signals thereof to change.
The O.sub.2 sensors, however, differ in characteristics depending upon the individual unit and also greatly vary in temperature characteristics. Therefore, in order to control the air-fuel ratio over a wide range of temperatures of the engine, while suppressing control errors that may stem from individual characteristics, a particular contrivance must be provided to process the output voltage of the O.sub.2 sensor. One method is to vary or control a reference voltage for comparison. Namely, the output voltage of the O.sub.2 sensor is compared with the reference voltage by a comparator, to discriminate whether the air-fuel ratio at the present moment is on the rich side or on the lean side relative to the stoichiometric condition. In this case, the reference voltage for comparison can have variable values responsive to a maximum value in the output voltage of the O.sub.2 sensor.
The present inventors have already proposed a method for controlling the air-fuel ratio by using a variable reference voltage dependent upon the maximum and minimum value in the output voltage of the O.sub.2 sensor (U.S. application Ser. No. 276,996). In this method, the variable reference voltage V.sub.R of the comparator is obtained from the equation of V.sub.R =V.sub.MAX .multidot.K.sub.1 or V.sub.R =(V.sub.MAX -V.sub.MIN).multidot.K.sub.2 +V.sub.MIN where V.sub.MAX is the maximum value, V.sub.MIN is the minimum value of the output voltage of the O.sub.2 sensor, and K.sub.1 and K.sub.2 are constants.
However, when using a semiconductor type O.sub.2 sensor, such as a titania element type, which transduces the change in the oxygen concentration to the change in its inner resistance, the above-mentioned control method set forth by the present inventors cannot precisely control the air-fuel ratio in response to the wide change in temperature of the O.sub.2 sensor was because the inner resistance of the semiconductor type O.sub.2 sensor greatly changes along with changes in its temperature. As is well known, the resistance of a semiconductor element increases when the temperature thereof decreases, and vice versa. Therefore, if a constant dc voltage is applied across the semiconductor type O.sub.2 sensor and a resistor having a fixed resistance connected so that the O.sub.2 sensor is made positive with respect to the resistor, the maximum value and minimum value of the voltage derived from the junction of the O.sub.2 sensor and the resistor change depending upon the O.sub.2 sensor's temperature, as follows. During low temperature, not only the minimum value but also the maximum value of the derived voltage change to approach to zero. Contrary to this, during high temperature, not only the maximum value but also the minimum value of the derived voltage change to approach the applied constant voltage. Therefore, precise air-fuel ratio control cannot be executed if the reference voltage for the comparison is merely varied in response to the maximum value of the output voltage from the O.sub.2 sensor, or to the difference between the maximum value and the minimum value of the output voltage, by using the above equation of the first degree. As a result, the air-fuel ratio is controlled to a ratio on the rich side with respect to a desired ratio at a low temperature of the O.sub.2 sensor, or the exhaust gas and vice versa.
Furthermore, if the temperature of the O.sub.2 sensor or the exhaust gas is extremely high, for example, above 750.degree. C., the air-fuel ratio will not be precisely controlled by the feedback loop, and is controlled to a ratio on the lean side relative to a desired ratio. Thus further overheats the catalytic converter and extraordinarily decreases the engine torque.